A permanent-magnet field synchronous motor comprising a rotor body having a cylindrical outer surface and a plurality of permanent magnets as a field system attached on the outer surface of the rotor body and arranged in a circumferential direction thereof has been known. In this kind of motor, each of the permanent magnets is magnetized so that magnetic poles are formed on the inside and outside surfaces thereof, respectively, and that the magnetic poles on the outside surfaces of the permanent magnets are alternately different in the circumferential direction.
When using such a permanent-magnet field synchronous motor as a servo motor, it is necessary to reduce fluctuation of a torque produced in the motor as much as possible in order to smoothly rotate a rotor assembly. Therefore, it is desirable that a characteristic of electromotive force produced in the windings of the stator changes sinusoidally in relation to a time. For this, as shown in FIG. 1, a construction in which a gap defined between the outside surfaces of the permanent magnets 12 fixed to the cylindrical outer surface of the rotor body 11 and the cylindrical inner surface of the stator 13 changes sinusoidally in accordance with rotation of the rotor body 11, has been adopted for a motor.
In the motor having such a construction, however, since the inner surface of the stator 13 is substantially a cylindrical surface, it has been necessary to give the outside surface of each of the permanent magnets 12 a specially curved shape in order to obtain such a gap sinusoidally changeable between the cylindrical inner surface of the stator and the permanent magnets. Generally, the permanent magnet has been made by a press sintering. When making a permanent magnet having such a specially curved shape, it is necessary to make a press mold having a specially curved concavity. Therefore, processing of such a permanent magnet is difficult. Further, a grinding process of the outside surface of such a permanent magnet, performed after molding thereof, is also difficult. Therefore, the processing accuracy of the permanent magnet is decreased. Furthermore, since such a permanent magnet has a thickness which becomes gradually thinner toward the opposite ends thereof in a circumferential direction, a pressing force tends to be applied unevenly to the permanent magnet, and thus an internal stress in the permanent magnet is not uniformly distributed. Consequently, the motor has a disadvantage in that the permanent magnet, being hard and brittle in general, is apt to be easily broken.